In a system for transmitting two level data bursts on a fiber optic (FO) bus, a circuit for turning on a light emitting diode to an excitation level induced by a current I.sub.B corresponding to the knee of the diode characteristic curve and increasing the emitted light in response to a second increasing excitation current over a second portion of the curve beyond the knee extending from excitation level I.sub.B to excitation level I.sub.M. The circuit includes first logic for exciting the diode by the first current, a light responsive device for determining when the emitted light reaches the intensity represented by the knee of the curve and induced by I.sub.B, and logic for reproducibly storing I.sub.B. Second logic responds to the excitation of the diode to the knee of its curve to further excite the diode with the second excitation current until the diode emits a desired light intensity induced by I.sub.B +I.sub.M. Logic is provided for reproducibly storing a signal I.sub.M. Switching logic supplies the current I.sub.M to the diode in response to the data bursts and supplies I.sub. B to the diode substantially concurrently with the data bursts. In the absence of the data bursts the current level I.sub.B is reduced to substantially zero.